Superjunction structure is a structure composed of alternately arranged N-type pillars and P-type pillars. A superjunction MOS transistor is formed by replacing the N-drift region of a VDMOS (Vertical Double-diffused MOSFET) device with a superjunction structure. By using a low-resistivity epitaxial layer, a superjunction MOS transistor may achieve a much lower on-resistance than a conventional VDMOS device while maintaining the same reverse breakdown voltage.
The distribution of N-type impurities in the N-type pillars, the distribution of P-type impurities in the P-type pillars, and the matching between the distributions of N-type and P-type impurities in the alternately arranged N-type and P-type pillars in a superjunction structure will affect the properties of the superjunction semiconductor device, including its reverse breakdown voltage and current handling capacity.
Generally, the alternately arranged N-type pillars and P-type pillars in a superjunction semiconductor device adopts a design of optimized electric charge balance so as to obtain a maximum reverse breakdown voltage, but in such devices, the current handling capacity is insufficient.
One method to improve the current handling capacity is to have the doping concentrations of P-type impurities in the P-type pillars in a superjunction structure unevenly distributed in the direction perpendicular to the surface of the substrate (i.e. in the vertical direction), while keeping the doping concentrations of N-type impurities in the N-type pillars evenly distributed. If the widths of the P-type and N-type pillars are equal to each other, then have the concentration of P-type impurities in the upper part of the P-type pillars higher than the concentration of N-type impurities in the N-type pillars, and have the concentration of P-type impurities in the lower part of the P-type pillars lower than the concentration of N-type impurities in the N-type pillars. Based on the above method, Infineon Technologies proposed a detailed solution to divide each P-type pillar in the superjunction structure into six sections along the vertical direction, and let the concentrations of P-type impurities in the six sections from the top down be respectively 30%, 20%, 10%, 0%, −10% and −20% higher than the concentration of P-type impurities in an optimized electric charge balance.
Currently, manufacturing methods of superjunction structure in a superjunction semiconductor device can be overall classified into two types. The first type is to either form an epitaxial layer of one doping type with a certain thickness and then perform lithography and ion implantation in certain regions of the epitaxial layer to form pillars of another doping type, or form an undoped epitaxial layer with a certain thickness, and then perform lithography and ion implantation to form N-type and P-type pillars in the epitaxial layer; the above step is repeated for a few times to form N-type and P-type pillars with a desired thickness. The second type is to etch trenches in a region of one doping type, and then perform trench filling, or epitaxy or ion implantation to the trenches to form pillars of another doping type for one-time.
The above described superjunction structure with improved current handling capacity requires a variation in the impurity concentrations distributed in the P-type pillars. But depending on the existing art, the method of the second type is impractical, and the method of the first type has shortcomings of high process costs, long manufacturing time and great difficulty in production control.